Projection displays for large screen televisions and monitors may use LCoS (Liquid Crystal on Silicon) chips to create an image. The image is created by directing light from a powerful lamp onto the surface of a LCoS chip, and operating the LCoS chip to control, with very fine resolution, the polarization of the reflected light, and passing the reflected light through a polarizer onto a display screen. At each pixel of the LCoS chip, the polarization of the reflected light may be changed depending on the state of the pixel. The light reflected by the LCoS chip is directed into a polarizer, and light of proper polarization is passed through a polarizer the screen, while light of improper polarization is reflected away from the screen, thereby differentiating the light and dark pixels in the image. The reflected light then passes through a projection lens to a screen.
A typical LCoS Projector of the prior is illustrated in FIG. 1, which illustrates in schematic form a traditional projector setup 1 utilizing one LCoS chip to create a projected image. In this system, a lamp and reflector assembly 2 provide the powerful light source, producing and directing light beams 3 into integrating rod 4, where light beams 3 are homogenized. Light beams 3 then exit the integrating rod and pass through color wheel 5. Color wheel 5 is rotated by a motor, which is governed by electronics, both not illustrated. The color wheel is thereby controlled so that it causes light beams 3 to pass through a specific color filter at an instant in time, therefore filtering only a specific color of light at that time. Light beams 3, now a certain uniform color (typically red, green or blue), next pass through optics 6.
Optics 6 focuses light beams 3 on polarizing beam splitter 7. Polarizing beam splitter 7 is positioned such that the polarizing surface is at a forty-five degree angle to the optical axis of optics 6. The polarizing beam splitter is also positioned at a forty-five degree angle to display surface 8 of LCoS chip 9. Polarizing beam splitter 7 divides light beams into two components: beams of light that have S polarization and beams of light that have P polarization. The beam splitter does this by reflecting the beams of light that have the opposite polarization than does the polarizing beam splitter and allowing the beams of light that have the same polarization as the polarizing beam splitter to pass through the polarizing beam splitter unaffected. For example, if polarizing beam splitter 7 is an S polarizing beam splitter, then light beams 3 are divided into light beams 10 having P polarization and light beams 11 having S polarization. Light beams 10 reflect off polarizing beam splitter 7 at roughly a ninety degree angle to light beams 3, and project onto LCoS chip 9, while light beams 11 pass through polarizing beam splitter 7. Light beams 11 may either be lost (not used to display an image on a screen), or may be projected back onto LCoS chip 9 through the use of polarization recovery optics (not illustrated).
LCoS chip 9 is controlled by electronics (not illustrated) that govern the state of the pixels on display surface 8 of the LCoS chip. The electronics control the image displayed on display surface 8 by selectively turning pixels on the display surface “on” and “off”. If LCoS chip 9 is a monochromatic display (capable of only displaying black or white), the pixels on display surface 8 can only be in an “on” or “off” state. If the chip is not a monochromatic display, that is, the chip can display grayscale, then the pixels on display surface 8 can occupy states between the fully “on” and fully “off” positions.
Light beams 10 then reflect off LCoS chip 9 at a roughly 180 degree angle to the incident light beams, and the state of the polarization of these light beams (originally P polarization) depends on the state of the pixels on display surface 8. For example, LCoS chip 9 may be designed such that light beams that reflect off pixels that are “on” have their polarization state rotated by ninety degrees. Light beams that strike “off” pixels reflect with their polarization state unaffected. Thus, light from light beams 10 that strikes “on” pixels is reflected back from display surface 8 with its polarization rotated from S to P. These light beams, since they now have the same polarization as the polarizing beam splitter, pass through the polarizing beam splitter to projection optics 12, which focus light beams 13 onto display screen 14 (not illustrated). Optics 12 are positioned such that the optical axis of the optics is perpendicular to display surface 8 of LCoS chip 9. Light from light beams 10 that strikes “off” pixels is reflected back from LCoS chip 9 with its polarization unchanged. When these light beams 15 strike polarizing beam splitter 7, the light beams reflect off the polarizing beam splitter at a roughly ninety degree angle to the incident light beams and pass back through optics 6. If LCoS chip 9 is a grayscale-capable chip, then light from light beams 10 may also reflect off partially “on” pixels, and thus undergo a polarization rotation between zero and ninety degrees. The light reflected from partially “on” pixels is partly transmitted through polarizing beam splitter 7 and partly reflected by it.
Through this pixel and light polarization manipulation, images on the LCoS chip are projected onto the screen. “On” pixels in the LCoS chip's image reflect light that passes through the polarizing beam splitter, and thus create a bright area in the projected image. “Off” pixels in the LCoS chip's image reflect light that the polarizing beam splitter blocks from reaching the display screen, creating a dark area in the projected image. Light that reflects off partially “on” pixels is used to create shades between the two extremes; the degree of how “on” the pixel is determines how light the shade is.
Images displayed by LCoS chips are magnified around one hundred times (100×) when they are projected on to a display screen. Therefore, any problems with defect pixels on the LCoS chip are going to be noticeable and annoying to a projector viewer. During the life of the system, it is possible that process defects in the LCoS chip may cause visual defects to appear in the projected image. Pixels may lose the ability to switch completely on and off before a display has reached the end of its lifetime. Some LCoS chips have defects after manufacturing which are not related to lifetime and are simply defects on the chip. Defects may lead to failure of pixels to operate. If a pixel is stuck in a permanent “on” or “off” state, it will appear on the screen as a permanent, non-moving white or black spot, respectively. These defects will cause the projected image to have artifacts that are not part of the intended image. This degrades image quality and is undesirable in an end product. FIGS. 2a and 2b illustrate this degradation of the image displayed to the viewer. In FIG. 2a, the display screen 14 shows an image which includes a black dog, but due to a single pixel on the LCoS chip which is stuck permanently on, a single pixel that should be black is instead illuminated, creating a noticeable visual artifact 16 on the image. In FIG. 2b, the display screen 14 shows an image which includes a white dog, but due to a pixel on the LCoS chip which is stuck permanently off, a single pixel that should be white is not illuminated, creating a noticeable visual artifact 17 on the image. These artifacts appear permanently on the display screen because they derive from physical defects in the LCoS chips. Systems that utilize only one LCoS chip to produce an image or portion of an image have no way to correct or ameliorate the effects that bad, non-functioning pixels have on an image.